Tuesday, December 30, 2014

Cleaning Up: How dredging is cleaning up Ridgefield’s Lake River

By Joyce Mercuri, Diana Smith, and Craig Rankine, Toxics Cleanup Program

How do you remove toxics from a sensitive environment, including a national wildlife refuge? This year, Ecology and the Port of Ridgefield have been using advanced cleanup techniques to remove pollution at the Pacific WoodTreating (PWT) cleanup site.

Recently, work shifted from Carty Lake to Lake River. The Carty Lake cleanup was finished in late October (learn more here and below). Meanwhile, the port’s contractor started cleaning up Lake River sediments in late October.

PWT’s operations polluted Lake River sediments near the port property with dioxins, pentachlorophenol (PCP), cresols, and polycyclic aromatic hydrocarbons (PAHs). Dioxin is the main contaminant of concern.

Removing these contaminated sediments from the river is the most effective way to reduce risk to human health and improve the environment for fish and animals that eat fish. However, ensuring that dredging doesn’t harm water quality or spread contamination is tricky. The port and Ecology are working hard to ensure dredging protects the surrounding environment.
Lake River dredge area

Designing “precision” dredging

The port, under Ecology’s oversight, chose precise dredging technologies. Precise methods help minimize sediment disturbance and keep contamination from spreading or affecting water quality.

Careful planning was used to select the dredge area up front, which saves time and money during construction. The dredge area was identified by extensively sampling the river bottom to find out how widespread and deep contamination is.

Engineers then used computer mapping and statistical methods to plan the exact outline and depth of the area to be dredged. The dredge area is shown in green and dark blue on the map. The light blue area outside of the dredge area is less contaminated and will be covered with one foot of clean sand.

Then, project engineers used very high resolution underwater sonar surveying to make a detailed map of the river bottom in the dredge area. From this, they created a grid map showing where each dredge bucket would be placed and how deep it needs to go to get the contamination out. After dredging, sonar surveying is done again to make sure dredging goes to the planned depth.

Grid map showing dredge bucket areas (tiny squares)

Preparing to dredge

Removing pilings with a vibratory extractor
Once workers got to the site they removed old pilings and other debris that could get in the way of dredging equipment. Debris can keep the dredge bucket from closing, which can let contaminated sediments leak out. Debris can also make the dredge bucket disturb and spread contaminated sediments.

In October, the port’s contractor pulled out 121 wood pilings, some logs, and about 180 tons of concrete, asphalt, and debris.

Dredging the right sediments

This dredging equipment is designed to avoid releasing or spreading contaminated sediments. During dredging:
  1. A large excavator with a specialized clamshell bucket is placed onto a barge that is anchored over the dredge area. Rather than using a bucket dangling at the end of a cable, the clamshell bucket is attached to a fixed arm so the operator can precisely place the bucket. 
  2. An equipment operator carefully places the open bucket on the area to be dredged.
  3. Then, the two halves of the bucket slowly close and seal. This prevents sediments from leaking out while the bucket containing water and sediment is brought to the surface.
  4. The first pass over the area collects most of the contaminated sediments.
  5. The equipment goes over the area a second time to remove any loose sediment that the first pass could have disturbed. The operator also makes sure they reached the design depth for that 
Empty bucket being lowered into Lake River and closed bucket coming out with contaminated sediments
Computer software for dredging and high resolution global positioning system (GPS) devices attached to dredging equipment help ensure precision dredging. With these tools, the dredge operator can essentially see underwater to ensure each scoop of the bucket removes sediments from the right area and at the right depth. Engineers oversee this in real time.
Operator in excavator over Lake River using software to make sure he's placed the bucket correctly

Disposing of contaminated sediments

The bucket collects water along with sediments. The operator carefully releases this water into a bin on the dredging barge. Since it might be contaminated, this water is pumped to a treatment system on the port property. The operator also places the sediments into a barge that has been sealed to prevent sediments or water leaking back to the river. Watch a video
Releasing water to container, then releasing sediments to another container on the barge
Once the barge is full, it is towed to the end of Division Street near the sediment handling and water treatment area. Excavators load the sediments from the barge into dump trucks and they are taken into a covered handling area. In order to be dry enough to be accepted at the landfill, some cement needs to be added to the sediments. Once they are dry enough, the contractor loads them into trucks and takes them to the nearby landfills.
Unloading sediments from the barge, then transferring dried sediments out of the handling area

Restoring the area and finishing Lake River cleanup

The port is almost finished with dredging contaminated sediments from Lake River. The port has started placing clean sand over dredged areas and areas that contain lower levels of contamination (see light blue areas on the map above). They’ve also started placing “fish mix” sized rocks to stabilize and restore the shoreline.
Crane operator grabbing sand for the riverbed and equipment placing fish mix rocks on shoreline
To finish this part of the cleanup, the port will finish the dredging, as well as stabilizing and restoring the shoreline. They will also plant native plants and trees along the embankment. This work will continue through early spring 2015.

The port is doing the current cleanup under a 2013 legal agreement with Ecology. Ecology and the port are funding the cleanup.

Finishing the Carty Lake cleanup
Carty Lake plant restoration

The port used a different technique to clean up contaminated sediments in Carty Lake. You can read about that here. Now, native grasses planted along the Carty Lake shoreline have started to sprout. Landscape crews also planted wetland plants. They will finish planting the banks in the spring.


1 comment:

Chuck Gruenenfelder said...

Nice article. Very informative.